Environmental protection agencies in many countries have enacted strict regulations for diesel exhaust particulate matter (PM) and NOx emissions. With current technologies, it is difficult to meet such regulations unless diesel particulate filter (DPF) and NOx reduction devices are installed in a vehicle exhaust system. Thus, significantly more complex exhaust systems have been installed in diesel vehicles manufactured in 2007 or later. For a typical heavy-duty diesel truck to meet 2010 US emission standards, exhaust aftertreatment devices, such as a diesel oxidation catalyst (DOC), a DPF, and a selective catalytic reduction (SCR) catalyst with urea injection or a NOx adsorber may be installed.
A DPF removes diesel particulate matter based on a filtration mechanism. Thus, while exhaust gas moves through the DPF, particulate matter is removed from the exhaust gas and stored in the filter. Over time the passage of exhaust gas through the pores of a DPF is progressively blocked, and the pressure required to maintain the exhaust gas flow increases. This pressure, which is the pressure higher than the exhaust must work against, is called “back pressure.”
As a DPF operates it removes particulate from exhaust gas, and back pressure in the exhaust system increases. As the DPF is increasingly soiled, the back pressure will eventually increase to a point significantly greater than the back pressure of a clean DPF, particularly if pressure measurements are taken upstream of the DPF. Beyond a certain limit, excess back pressure can increase exhaust temperature, carbon monoxide emission, and PM.
Because of the detrimental effects of excessive exhaust back pressure on an engine, DPF's are periodically regenerated by removing trapped particulate matter. By regenerating a DPF loaded with soot, the back pressure in an exhaust system can be reduced to a normal level.
To study and evaluate the performance of engines and aftertreatment devices, engineers and researchers are interested in measuring gaseous emissions (CO, THC, NOx, CO2, etc.) under the varying back pressure conditions observed when a DPF is used in an exhaust system. However, the environmental conditions present in exhaust systems using a DPF present several problems for conventional gas analyzers and emissions benches, as discussed below.
For the purpose of the present disclosure, the term “emissions bench” refers to instrumentation that is configured to analyze one or more properties of exhaust gases generated by a combustion source, such as an internal combustion engine. For example, an emissions bench may include one or more instruments configured to measure or determine at least one of the identity, mass, and concentration of one or more components (e.g., O2, CO2, CO, NOR, and hydrocarbons) of such exhaust gases.
Conventional gas analyzers and emission benches are generally designed to operate in a low back pressure environment, such as in the range of 0 to 30 kPa above ambient air pressure. Once the back pressure exceeds 30 kPa above ambient air pressure or drops below ambient air pressure, the sample flow into the instrument may be beyond the control of the sampling system used to provide samples of the exhaust gas to a gaseous analyzer or emissions bench. This can lead to improper operation of the gaseous analyzer and other instruments in an emissions bench.
FIG. 1 is a schematic of a conventional gaseous measurement system utilized in exhaust emission measurement. As shown, system 100 includes a probe 103 disposed within tailpipe 101 of a vehicle. Probe 103 samples exhaust 102 flowing through tailpipe 101. The resulting sample is pulled through particulate filter 104 (e.g., a high efficiency particulate air filter) via sample line 105 and vacuum pump 106, and ultimately enters emissions bench 107.
While the system shown in FIG. 1 is effective in some circumstances for analyzing exhaust, the sample flow rate may be sensitive to the inlet pressure. If the inlet pressure to the probe 103 is outside of the design inlet pressure for the system, the sample flow rate may be out of the designed sample flow rate. As a result, the instruments within emissions bench 107 may not operate correctly.
To address this issue, several modification kits have been developed. One example of such modification utilizes a bypass to lower the sample inlet flow to the analyzing instruments when a high back pressure is detected. That is, when a high back pressure condition is detected, the system maintains the inlet pressure within a designed range by increasing a bypass flow upstream of the emission bench.
While the use of a bypass can address the pressure problem encountered during a high back pressure condition, the bypass impacts the residence time of a sample flowing through the system. Specifically, as back pressure increases, sample residence time decreases because more of the sample flow is vented through the bypass, e.g., via a bypass pump. In contrast, sample residence time increases under a lower back pressure condition. Due to this variance, the residence time of the sample may not correlate with a delay time that is stored in the emissions bench. This can bias the results of measurements taken with instrumentation within emissions bench 107, and may cause other measurement errors.